1. Field of the Invention
This invention relates to man-made inorganic oxide fibres. The invention also relates to products formed from such fibres.
2. Description of the Related Art
Inorganic fibrous materials are well known and widely used for many purposes (e.g. as thermal or acoustic insulation in bulk, mat, or blanket form, as vacuum formed shapes, as vacuum formed boards and papers, and as ropes, yarns or textiles; as a reinforcing fibre for building materials; as a constituent of brake blocks for vehicles). In most of these applications the properties for which inorganic fibrous materials are used require resistance to heat, and often resistance to aggressive chemical environments.
Inorganic fibrous materials can be either glassy or crystalline. Asbestos is an inorganic fibrous material one form of which has been strongly implicated in respiratory disease.
It is still not clear what the causative mechanism is that relates some asbestos with disease but some researchers believe that the mechanism is mechanical and size related. Asbestos of a critical size can pierce cells in the body and so, through long and repeated cell injury, have a bad effect on health. Whether this mechanism is true or not regulatory agencies have indicated a desire to categorise any inorganic fibre product that has a respiratory fraction as hazardous, regardless of whether there is any evidence to support such categorisation. Unfortunately for many of the applications for which inorganic fibres are used, there are no realistic substitutes.
Accordingly there is a demand for inorganic fibres that will pose as little risk as possible (if any) and for which there are objective grounds to believe them safe.
One line of study has proposed that if inorganic fibres were made that were sufficiently soluble in physiological fluids that their residence time in the human body was short; then damage would not occur or at least be minimised. As the risk of asbestos linked disease appears to depend very much on the length of exposure this idea appears reasonable. Asbestos is extremely insoluble.
As intercellular fluid is saline in nature the importance of fibre solubility in saline solution has long been recognised. If fibres are soluble in physiological saline solution then, provided the dissolved components are not toxic, the fibres should be safer than fibres which are not so soluble. The shorter the time a fibre is resident in the body the less damage it can do.
Such fibres are exemplified by the applicant's earlier International Patent Applications WO93/15028 and WO94/15883 which disclose saline soluble fibres usable at temperatures of 1000.degree. C. and 1260.degree. C. respectively.
An alternative line of study has proposed that hydratable fibres that lose their fibrous nature in body fluids may offer another route to "safe" fibres in that the shape and size of the fibres may be what is causing damage. This route is exemplified by European Patent Application Nos. 0586797 and 0585547 which are aimed at providing silica free compositions and which disclose two calcium aluminate compositions (one containing 50/50 wt % alumina/calcined lime and the other disclosing 63/30 wt % alumina/calcined lime with additions of 5% CaSO.sub.4 and 2% other oxides). Such fibres hydrate readily so losing their fibrous nature. Asbestos does not hydrate and seems to maintain its fibrous shape in body fluid effectively indefinitely.